Using conduit

For outdoor wiring—hooking up a
hot tub, adding outlets to a deck,
powering a shed—installing conduit
makes a lot of sense, and many
times it’s required. Conduit even has
its uses inside, wherever wires
would otherwise be exposed and
could be damaged. And if you have
a circuit you may want to extend
someday, conduit will enable you to
pull additional wires.

We’re going to show you how to
install plastic (PVC) rigid conduit
rather than metal conduit. Plastic
conduit is less expensive, lighter and
much easier to work with. Here are some
great pro tips from commercial electricians to help you make wiring runs with conduit.

Schedule 40 vs. 80

Is one better?

Schedule 40 conduit is cheaper and has a larger inside diameter, so it’s easier to pull wires
through it. The plastic on Schedule 80 is thicker, but the conduit has the same outside
diameter as 40, so the inside diameter is smaller. Always install Schedule 80 conduit in
high-traffic areas or any other areas where it could get damaged, like behind your woodpile. By the way, the fittings (such as adapters and turns) are the same for each type.

Buy THHN wire

Best for pulling

THHN (thermoplastic high heat resistant
nylon-coated) is the best
wire for pulling through conduit.
Other types of wire have a sticky
rubber sheathing that makes them
almost impossible to pull.
Stranded THHN is used on most
commercial jobs—it’s more flexible
than solid wire, which makes it
easier to pull, and it doesn’t spring back when you push it into the box.

Larger conduit and bigger boxes

Bigger is easier to work with

Larger boxes and 3/4-in. conduit make it much easier to pull wires and to fit devices, connectors and wire nuts in without cramming them.

Install 3/4-in. conduit instead of
1/2-in. if (1) you need to pull more
than three wires through one section
of conduit; (2) there’s any
chance you’ll add wires in the
future; or (3) if you have a long
and winding run. The 3/4-in. conduit
doesn’t cost that much more,
and it’s a heck of a lot easier to
pull wire through. Whatever size
conduit you use, don’t fill it more
than 40 percent with wires.

Single-gang electrical boxes
will work, but if you have two or
more conduit sections connecting
to one box, buy double-gang. The
male connectors on the ends of
the conduit take up quite a bit of
room inside the box, leaving little
room for devices. GFCI receptacles
and other large devices, like
dimmers, fit better in deeper boxes (2-1/8 in.).

Deburr with a utility knife

Clean up rough cuts

If you do end up with a rough
cut, don’t forget to deburr the
inside of the conduit. Burrs
can damage the insulation on
the wires. There are a lot of
fancy deburring tools out
available, but spining
a utility knife on the inside of the conduit will smooth it out just fine.

Conduit in a trench

If you’re running rigid PVC conduit,
most trenches need to be 18 in.
deep, but ask your electrical inspector
how deep to dig the trench for
your specific project. You can run
Schedule 40 in a trench, but use
Schedule 80 wherever the conduit
comes up out of the ground (see
“Schedule 40 vs. 80” above).
Assemble all the conduit first and
plop the whole thing into the trench
when you’re done. Much easier and cleaner than working in a trench!

Conduit doesn't need primer

Joining PVC

Measure as carefully as
you can so you can avoid dry-fitting
your connections.
Unlike PVC plumbing pipes,
PVC conduit and fittings can
be difficult to pull apart once
you shove them together.
And always wear gloves,
unless you want to spend
half the evening picking glue off your hands.

Some PVC pipes require
primer, but you don’t need
to use primer when gluing
conduit and fittings. Home
centers usually sell the
appropriate cement near the the conduit and fittings.

Mount the box, then the conduit, then the box

Don't force the conduit

Installing boxes one at a time makes it easier to fasten the connectors,
nuts and bushings (see below) to the box first and
then glue the conduit to the connector.

It’s tempting to start by attaching all the boxes to
the walls and ceiling first and then run the conduit,
but don’t do it. It’s easier to secure one box
and then run the conduit from that box to the
next one. Fasten the second box to the wall or
ceiling after you fasten it to the conduit. Then you
won’t have to fight the conduit trying to bend it
into position. This is especially important if you
have two boxes in close proximity because it’s
difficult to bend short sections of conduit.

Metal hangers work best

Support conduit every 3 ft.

Use metal hangers even with PVC conduit;
they hold up better than plastic. Choose the
single-hole type. One screw is enough support, and compared with the
two-hole strap, installation will go twice
as fast.

Your job will look better if you install the
kind of hanger that offsets the conduit the
same distance from the wall as the knockout
on your boxes. For 1/2-in. through 1-in.
conduit, the maximum spacing between supports is 3 ft.

Cut it with a circular saw

Metal blades work best

There are lots of ways to cut
PVC conduit, but a circular
saw fitted with a metal blade
gives you a smooth, fast,
burr-free cut. If you don’t
have a metal blade, a regular
construction blade will do the
job, but you may have to
deburr the end of the conduit after you cut.

Install metal locknuts

Avoid stripping threads

Keep elbow totals no more than 360 degrees

Avoid too many curves

If you have a long run with a
whole bunch of twists and
turns, consider splitting up
the span with junction boxes.
Every elbow you install
makes pulling wire more difficult.
And installing turns
totaling more than
360 degrees (four 90-degree
elbows) is not allowed on one run.
Pros rarely go beyond 180
degrees because it’s easier to install
an additional box and pull the wire a shorter distance.

Drill a hole to let water out

Assume the worst

There’s still a strong possibility that water will get inside your weatherproof
box. Drill a 1/4-in. hole in the bottom of the box, so if water gets in, it can get out. You can drill the hole before or after you install the box.

Hook on Old Wires to Pull New Ones

If you’re adding wires to existing conduit and have to pull them a long distance,
hook the new wires to an existing one—including a replacement wire for the one
you’re using—and pull them through that way. You’ll have to buy extra wire, but you’ll save a lot of time and frustration.

Pulling wires

Pro tip

It’s easy to push wires short
distances, but if it’s necessary
to pull them a long distance
with fish tape, here’s
how pros tie them
on: First strip 4 in. of
sheathing off two wires.
Then cut half the
strands off the two exposed
wires (less bulk to pull
through). Next loop the
remaining exposed wires
through the eyelet of the
fish tape. Finally, wrap
all three wires in electrical
tape all the way up to the eyelet of the fish tape.

Bushings protect wires

Avoid snags

Even if you deburr the end of your
pipe, you can still damage wires
when pulling them past conduit
edges. A bushing provides a nice
rounded, smooth surface for the
wires to slide by. It’s cheap insurance,
and your electrical inspector will be impressed.

Use weatherproof boxes outdoors

Keep water out

Install weatherproof boxes (sometimes
called bell boxes) outside. Unlike regular
boxes, weatherproof boxes usually have
threaded knockout holes to create a
water-resistant connection. Many come
with caps to plug the hole you don’t use.
Make sure the box you buy has holes where you need them.